FIG. 10 is a block diagram showing an example of the arrangement of a conventional lens-exchangeable digital still camera.
Referring to FIG. 10, a camera operation switch 43 comprises a main SW and release SW. When a photographer has operated the camera operation switch 43, a total control circuit 44 detects a change in state of the camera operation switch 43, and starts power supply to other circuit blocks.
An object image within a photographing frame range forms on an image sensing unit 34 via a main photographing optical system 33, and the image sensing unit 34 outputs an electrical signal to an A/D converter 35. The electrical signal is converted in turn into a predetermined digital signal for each pixel, and the digital signal is sent to a process circuit 36. The process circuit 36 generates R, G, and B color signals based on pixel data, and periodically transfers the generation results for each frame to a video memory 41 via a memory controller 37 while the photographer performs pre-photographing operation. A monitor display 42 makes viewfinder display and the like based on the data transferred to the video memory 41.
On the other hand, when the photographer has made photographing operation on the camera operation switch 43, pixel data for one frame output from the process circuit 36 are stored in a frame memory 38 in response to a control signal from the total control circuit 44 that detects a change in state of the camera operation switch 43. Subsequently, the data in the frame memory 38 is compressed by the memory controller 37 and a work memory 39 in a predetermined compression format, and the compression result is stored in an external memory 40. Note that the external memory 40 comprises a nonvolatile memory such as a flash memory or the like.
In order to display an image based on photographed image data, the memory controller 37 decompresses the compressed data stored in the external memory 40 to normal data for respective photographing pixels, and transfers the decompression result to the video memory 41. The monitor display 42 makes viewfinder display and the like based on the data transferred to the video memory 41.
An image sensing element used in such conventional lens-exchangeable digital camera uses microlenses shown in FIGS. 11A and 11B for respective photosensitive pixels of the image sensing element so as to improve photosensitivity for each pixel.
FIGS. 11A and 11B show the principle of shading (spatial sensitivity nonuniformity depending on the incident angle of incoming light from a lens) produced depending on the relationship between the microlens position and the angle of incoming light from the lens. Referring to FIGS. 11A and 11B, reference numeral 20 denotes a lens as a field lens; 21, microlenses; and 22, photosensitive portions of the image sensing element.
Since the microlenses 21 are provided for respective photosensitive portions 22 of the pixels of the image sensing element, even when each photosensitive portion 22 of the image sensing element has a narrow effective sensitivity region, marginal light can be effectively focused on the photosensitive portion 22.
However, when light rays that have passed through the lens 20 nearly perpendicularly strike the photosensitive portion 22 of the image sensing element, as shown in FIG. 11A, the incoming light rays focus on the photosensitive portion 22 of the image sensing element without posing any serious problem. However, when light rays that have passed through the lens 20 obliquely strike the image sensing element, as shown in FIG. 11B, only some incoming light rays become incident on each photosensitive portion in a region (peripheral portion of the image sensing element) separated from the optical axis of the lens 20 due to the optical relationship between the lens 20 and microlenses 21.
Such light amount drop is normally called white shading. This phenomenon becomes conspicuous as the pixel position on the image sensing element separates farther away from the optical axis of the lens 20.
A method of correcting white shading generated by a combination of the lens 20 and microlenses 21 on the image sensing element is disclosed in, e.g., Japanese Patent Laid-Open No. 6-197266. In this method, shading correction data is stored in advance in an internal memory of a lens, and is read out upon photographing. A reference voltage of an analog/digital conversion circuit is generated based on the readout data, and an analog sensed image signal is converted into a digital signal based on this reference voltage, thus implementing shading correction.
When the conventional lens-exchangeable digital camera uses the same optical system (e.g., an exchangeable lens system) as that in a conventional single-lens reflex silver halide film camera, it requires an image sensing element with a size considerably larger than that of an image sensing element of a digital camera that cannot use such optical system.
However, in such large-size image sensing element, the incident angle of light rays on the peripheral portion of the image sensing element becomes large depending on lenses. In such case, sensitivity nonuniformity (shading), i.e., a decrease in sensitivity on the peripheral portion of the image sensing element where the incident angle becomes large, is produced due to the microlens position, manufacturing errors of on-chip color filters, device structure of the image sensing element, and the like. Such problem will be explained below.
As shown in FIGS. 11A and 11B, a recent solid-state image sensing element comprises the microlenses 21 for focusing incoming light onto photodiodes (photosensitive portions 22 of the image sensing element) to improve sensitivity. However, when the incident angle of incoming light becomes large, the incoming light refracted by the microlens 21 is focused at a position separated away from the center of each photodiode, thus decreasing the sensitivity of that pixel.
Conventionally, an image sensing apparatus that makes shading correction in consideration of an increase in incident angle of incoming light on the peripheral portion of the image sensing element due to the characteristics of the lens that forms an object image is available, as disclosed in Japanese Patent Laid-Open No. 6-197266. However, in an image sensing apparatus system that can exchange lenses, the incident angle characteristics of incoming light rays to the image sensing element change drastically upon exchanging lenses, and shading correction cannot be accurately done.
Even when a single lens is used without exchanging lenses, if that lens is a zoom lens, since the exit pupil position of the lens changes when the photographing field angle has changed or when the lens is brought into focus to objects at different distances, the incident angle of incoming light into the peripheral portion of the image sensing element changes, and shading correction cannot be accurately done.
Furthermore, FIGS. 12A to 12C show the generation principle of color shading due to multi-layered type reflective infrared cut filters. FIG. 12A shows spectral transmission characteristics of the reflective infrared cut filters depending on the incident angles, FIG. 12B shows an example of spectral sensitivity characteristics of an image sensing element which comprises on-chip color filters, and FIG. 12C shows the sensitivity of a red filter pixel. FIGS. 12A to 12C exemplify an image sensing element which has RGB primary color type on-chip color filters. Note that the wavelength on the abscissa of FIG. 12A has the same level scale as that of the wavelength on the abscissa of FIG. 12B.
Assume that light which has passed through the infrared cut filters having the spectral transmission characteristics shown in FIG. 12A enters the image sensing element having the spectral sensitivity characteristics shown in FIG. 12B. In this case, as can be seen from FIGS. 12A and 12B, the spectral transmission characteristics of the infrared cut filters have no dependence on incident angle in the wavelength ranges of blue and green color filters, but have large dependence on incident angle in the wavelength range of a red color filter. The sensitivity characteristics of a signal output from the image sensing element are expressed by the product of the spectral transmission characteristics of the infrared cut filters and the spectral sensitivity characteristics of the image sensing element. Therefore, when the incident angle of incoming light is large, sensitivity drop of pixels having red color filters occurs, and color shading, i.e., a decrease in sensitivity of only pixels having red filters on the peripheral portion of the image sensing element is produced, as shown in FIG. 12C.